Generally, a flat panel display (FPD) has an advantage of saving space as it can be designed to be thin and be driven by a relatively low voltage. Well known FPDs include: a field emission display (FED), a vacuum fluorescent display (VFD), a liquid crystal display (LCD), and a plasma display panel (PDP).
Such FPDs are generally formed of a vacuum container having a pair of facing panels and a spacer for maintaining a gap between the panels. When the panels are sealed in a high vacuum state, the panel may be deformed or damaged by the pressure difference between the inner and outer sides of the panels. The spacer prevents such deformation and damage to the panels. In addition, the spacer maintains the cell gap between the panels to uniformly realize the brightness when an image is displayed by exciting phosphors. The spacer is generally formed through screen-printing. That is, a screen mask having a predetermined pattern of mesh holes and a panel on which the spacer is to be formed are first fixed on a printing device. Paste is provided on the screen mask and squeezed onto the panel through the screen mask. However, screen-printing has a limitation in precisely forming the spacer and in increasing the aspect ratio (i.e., the height with respect to the width).
Accordingly, in recent years, a photosensitive glass spacer has been proposed to solve the above problems. U.S. Pat. Nos. 5,894,193 and 6,149,484 disclose a field emission display having such a photosensitive glass spacer and a method for manufacturing the same. As taught by these patents, a photosensitive glass having a predetermined thickness is crystallized in a predetermined pattern, and the crystallized pattern is removed to form a single spacer frame assembly. However, the spacer may deteriorate the quality of the flat display, due to the following reasons.
First, when the light exposure for crystallizing the photosensitive glass is not fully realized, the crystallization on the opposite surface, which is not directly exposed to the light, is realized less than at the light-exposing surface during the heat-treatment process for baking the spacer. This causes the aspect ratio of the completed spacer to be reduced. This will be described in more detail with reference to the accompanying drawings. As shown in FIG. 8a, photosensitive glass 100 having a predetermined thickness (i.e., 1.2 mm) is formed in a predetermined pattern through a light exposing process whereby ultraviolet rays (UV) are emitted onto one surface 102 of photosensitive glass 100. Then, glass 100 is heat-treated to form selective crystallized portion 104 on photosensitive glass 100. Crystallized portion 104 is removed through an etching process to form a single spacer. During this process, when the light exposure is not fully performed, an opposite surface 106 of light exposing surface 102 of the glass is not sufficiently exposed to the ultraviolet rays, and the crystallization is not sufficiently realized on opposite surface 106. Therefore, as shown in FIG. 8b, the width of the upper and lower portions of spacer 108 becomes different, resulting in the reduction of the aspect ratio. Accordingly, to solve the above problems, the light exposure is performed for a sufficient time. However, when the thickness of the photosensitive glass is doubled, the light exposure time must be increased six times. This is time-consuming and deteriorates productivity.
Secondly, the spacer is designed not to discriminate as to the upper and lower portions. This structure makes it difficult for the spacer to be easily arranged on the panels as the patterns of electrode and phosphor layers are differently formed on the facing panels. For example, a cathode panel is provided with plural stripe-type electrodes and an anode panel is provided with a dot-type phosphor layer. Therefore, it is difficult to effectively arrange the spacer on the non-display area of the panels.
Thirdly, while a rectangular frame-type or cross-type spacer can be easily arranged, however to obtain the effective function of the spacer, the number of spacers should be increased, making it difficult to arrange the spacers. A rib- or sheet-type spacer can be arranged in the longitudinal direction of the panel, reducing the number of spacers. However, a special member for stably supporting the spacers becomes required.
The present invention provides a solution to the above-described problems.